Hi,
the subject line describes the project already
Suppose you have a file foo.msl. Somewhere in the file, you have the line 
INCLUDE bar.msl

Once you click save in the sequencer page, mhttpd crashes:
$ mhttpd
free(): double free detected in tcache 2
[1]    27590 abort (core dumped)  mhttpd


GDB helps shed some light on the problem:

#0  0x00007ffff76b057f in raise () from /lib64/libc.so.6
#1  0x00007ffff769a895 in abort () from /lib64/libc.so.6
#2  0x00007ffff76f39d7 in __libc_message () from /lib64/libc.so.6
#3  0x00007ffff76fa2ec in malloc_printerr () from /lib64/libc.so.6
#4  0x00007ffff76fbdf5 in _int_free () from /lib64/libc.so.6
#5  0x00000000004b8b41 in mxml_parse_entity (buf=buf@entry=0x7fffffffc2c8,
file_name=file_name@entry=0x7fffffffc710
"/home/luk/packages/mutrig_daq/online/foo.xml",
            error=error@entry=0x7fffffffcd24 "XML read error in file
\"/home/luk/packages/mutrig_daq/online/foo.xml\", line 2: bar.msl.xml is
missing", error_size=error_size@entry=256,
                error_line=error_line@entry=0x7fffffffce24) at ../mxml/mxml.c:1996
#6  0x00000000004b966d in mxml_parse_file
(file_name=file_name@entry=0x7fffffffc710
"/home/luk/packages/mutrig_daq/online/foo.xml",
            error=error@entry=0x7fffffffcd24 "XML read error in file
\"/home/luk/packages/mutrig_daq/online/foo.xml\", line 2: bar.msl.xml is
missing", error_size=error_size@entry=256,
                error_line=error_line@entry=0x7fffffffce24) at ../mxml/mxml.c:2041
#7  0x000000000041d9c2 in init_sequencer () at src/mhttpd.cxx:14321
#8  0x000000000040c2b6 in main (argc=<optimized out>, argv=<optimized out>) at
src/mhttpd.cxx:18028

Cheers
Lukas

P. S. This problem reminds me of the old joke: A man goes to his doctor and says
"Doc, it hurts when I do this" to which the doctor replies "Then don't do that".
However, I think, mhttpd should not crash even if you're not supposed to include
non-existent scripts in msequencer.